The field of the present invention relates to a telecommunications network. In particular, multiple upstream/downstream bandwidth allocations on a common hybrid (i.e. electrical and optical) telecommunications network are described herein.
Many telecommunications networks (carrying voice, data, video, or other information) include both optical network pathways and electrical network pathways, i.e., such networks are hybrid networks. Signals in the electrical portions of the network typically comprise radio-frequency (RF) electrical waveforms encoding the transmitted information, while signals in the optical portions of the network typically comprise propagating optical power modulated according to the RF amplitude of the equivalent RF electrical waveform. Both optical and electrical portions of a hybrid telecommunications network typically carry signals in either direction; such a network is bidirectional. A transition between optical and electrical network portions typically occurs at a network interface unit (NIU), wherein “upstream” signals received from the electrical portion of the network are converted to optical signals and transmitted to the optical portion of the network and “downstream” signals received from the optical portion of the network are converted to electrical signals and transmitted to the electrical portion of the network. The designations of “upstream” and “downstream” have become common usage and are used herein for convenience of description, however, these terms are not intended to be limiting and could in fact be replaced with “one direction” and “the other direction” or similarly generic terminology without altering the scope of the present disclosure or appended claims.
In telecommunications networks employing bidirectional electrical signal transmission in a common cable, electrical signal bandwidth typically is allocated into non-overlapping “upstream” and “downstream” RF frequency ranges, often with a “crossover” RF band separating them in which no signals are transmitted in either direction (designated “x-freq” in the drawings). By thus separating the upstream and downstream bandwidth allocations, interference between upstream and downstream signals is substantially avoided. Such interference is of particular concern in electrical portions of the hybrid network due to signal reflections at various components of the electrical portion of the network. Interference between upstream and downstream electrical signals can result in distortion of the signals to an unacceptable extent. In optical portions of the hybrid network, overlapping upstream and downstream RF frequency ranges are less of a concern, since signal reflections typically can be suppressed to a greater degree in the optical portion of the network than in the electrical portion. In addition, differing optical carrier wavelengths can be employed for the upstream and downstream optical signals, with wavelength-selective optical components substantially eliminating interference between the upstream and downstream signals.
FIG. 1 illustrates schematically a prior-art network interface unit (NIU). NIU 10 comprises an optical diplexer 102 and a radio-frequency (RF) diplexer 104. An optical fiber 12 serves as a bidirectional optical signal port and is connected to the optical diplexer 102. A cable 14 (e.g., coaxial cable) serves as a bidirectional RF signal port and is connected to the RF diplexer 104. The RF diplexer is configured and connected (i) to receive from the optical diplexer 102 an RF signal derived from an RF-modulated optical signal received from optical fiber 12 and (ii) to transmit the derived RF signal to the cable 14. The RF diplexer is also configured and connected (i) to receive from the cable 14 an RF signal and (ii) to transmit the received RF signal to modulate an optical signal transmitted by the optical diplexer to the optical fiber 12. The prior art NIU 10 can further comprise an RF amplifier 106 configured and connected to amplify the derived RF signal, or can further comprise a laser drive circuit 108 configured and connected to modulate a laser drive signal with the received RF signal to modulate the transmitted optical signal. An RF splitter 16 connects cables 14, 14a, and 14b, each of which can carry bidirectional RF signals.
Bidirectional RF signals are not transmitted at overlapping RF frequencies due to unacceptable levels of interference typically encountered. RF input signals are received into NIU 10 through cable 14 only within an input RF frequency range (often referred to as the “upstream” RF band), while RF signals are transmitted from NIU 10 through cable 14 only within an output RF frequency range (often referred to as the “downstream” RF band). The input and output RF frequency bands are non-overlapping, and are often separated by a range of RF frequencies that are used neither for receiving nor for transmitting RF signals to/from NIU 10 (referred to herein as the crossover range or band, or “x-freq” in the drawings). In this way interference between input and output RF signals (i.e. upstream and downstream RF signals) can be avoided. Standard RF frequency band allocations in current use are about 5-42 MHz for the “upstream” RF band (also referred to as the “CATV return band”) and about 54-870 MHz for the “downstream” RF band (with the crossover band being about 42-54 MHz). Other RF frequency ranges can be used. The observed asymmetry in bandwidth allocation between the standard upstream and downstream RF frequency bands has arisen due to historically greater demand for downstream transmission bandwidth versus upstream transmission bandwidth.